We're fans of Flex Grip, which allows you to clip the headset onto your earlobe much as you would a clip-on earring. As a result, the bud portion of the headset sits more securely in your ear. We were able to get a pretty snug fit, though if you have a particularly small ear (and canal), the hard plastic bud, which is covered by a foam pad, won't fit as well. The flexible boom microphone is exactly that--flexible--so you'll have no trouble fitting it to the contours of your face.
The X20--which also works with cell phones--isn't as fancy or quite as feature-laden as the company's step-up GameCom Halo 2 Special Edition ($49.99), which has a bidirectional microphone (the microphone in the X20 is unidirectional but does have noise-canceling technology). In other words, it doesn't offer quite the performance of the Halo 2 Special Edition, but the difference isn't great.
Like the Halo 2 Special Edition headset, the Plantronics GameCom X20's advantage over the standard, neckband-style Xbox Live model is that you can lie back in a couch or a lounge chair without displacing the headset. It's also eyewear-friendly. Bottom line: if you don't want to spend the extra dough on Plantronics's step-up model or Logitech's Cordless Headset, the X20 is a worthy compromise.